Sadly we have black fly on some of our runner bean plants. We have sprayed them but, more interestingly, they are being eaten by what we thought were caterpillars. Having googled the problem, indications are that the little insects are more likely to be syrphid fly (Hover Fly) larvae. They are up to 1cm long, have legs and are black with yellow stripes running across their bodies.
Just wondered if anyone else has seen these and can confirm my thoughts?